Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

mp4 for compatibility, literally click and play on 99% of devices, most OSes come preloaded with software to play mp4 files.

mkv for functionality, it's a much better format in general but as you mentioned, they're not tech savvy and it is possible that they may not gave VLC or any other 3rd party video player so they may face issues, I think it'd wiser to give them mp4 files.

And for encoding, you can either encode in H.265 or AV1, both are amazing, they'll help you save almost half of your storage, since they're way better than H.264, the quality will be indistinguishable but the files sizes will be halved.

.mp4 with H264: the most universal, and can be compressed to smaller sizes than you might think. Compatibility and compression will still vary depending if you use AAC or opus audio

.webm with VP9 and opus audio: better compression, not as universal. More open-standards based, maybe best balance of compression and compatibility

.mp4 or webm with AV1 and opus audio: probably best compression, also probably less compatibility than VP9, maybe depends what devices they use - good on new computers / phones / Android based TVs, more iffy on a wider or older range of devives. MP4 maybe friendlier than webm on newish Apple devices

The safest format I can think off would probably be mp4, with h264 for video streams and mp3 for audio streams. Unless you go for ancient technology basically anything should be able to open those files.

You could always package them with everything they need to watch, like VLC (which should be able to read most formats anyway). Not clear if they are all on Windows, but perhaps you could include the portable VLC version that doesn't require an install and make an obvious-named VLC playlist file for them to open.

as others have mentioned mp4 with h264 is almost certainly the most compatible. that being said, I transcode everything to hevc if I can't get it natively, and never have issues. my server literally cannot transcode. it does not have a GPU, and hevc plays natively on every target device I need. even works in browsers these days.

most people will still say h264 is best. but if you're limited on storage space or want to optimize streaming bitrate hevc works wayyy better than it did even just 1 or 2 years ago.

I'm mostly echoing what's already been said, but I have a preset in Handbrake for this, which works fine on most TVs I've tried from the last 10 years (possibly 15 by now) and therefore should have no problem running on any computer. I often (for work reasons) prepare video footage for looped playback on TVs and projectors at numerous places - so "TVs I've tried" is a larger number than it might initially sound like.

It's roughly along these lines (as I appear to have emailed someone about before):

"H264 mp4. 1920x1080. 25 or 30fps, or similar (appropriate to source material). Constant bitrate <=12mbps. 8mbps is generally universally compatible, though you should be able to get away with 10-12mbps on newer TVs with newer USB sticks.

AAC audio 192kbps, though lower is fine.

Use same samplerate as source (i.e. 44khz 48khz etc)

If you've got settings for encoding profile, Main and Level 4.0 should work.

If individual files are small enough (<4GB), format the USB stick as FAT32. Otherwise NTFS. EXT2 will work on a lot of TVs, but you'll have trouble with some computers. Exfat may work on newest tellys, but won't on anything more than a few years old, so safe option is not to use it."

The only file format that pretty much 100% guarantees support on most media hardware is h.264 in MP4 containers. With some encoder tuning you can make them decently small without loss of fidelity; people will notice bad encoding more than they will a slight loss in pixels. I would focus on making a really high quality 720p copy of the shows ans batch encoding them with handbrake (or finding good encoded copies on the usual places)

Depends haevily on the manufacturer.
But for me it was pretty much everything in mp4 (and newer probably also can do mkv) in H.264.

I'd avoid H.265, with the exception for very new and fancy tvs (usually OLEDs and higher end TVs from >2018.
Never do AV1.

Also keep in mind, that not every audio-codec has support.
Try to go stereo or (I think AC3).

Avc (h264) 8bits video, with AAC audio, hardcoded subtitles and .mp4 container.

That should be warrantied to work on every dumb device built this or last decade.

My old LG SmartTV seems most reliable at playing mkv files but I think mp4 is pretty standard.

I’ve found x265 is pretty universally supported in 2025. I’ve switched all of my downloads for Plex over to 265 and none of my users have reported issues. My users are not particularly tech savvy and have a ton of diversity in their devices from cheap ass smart TVs, to consoles, to various mobile devices and computers.

I think it’s fairly safe to start getting everything in 265 and the space savings is significant. Very easy to get 4K HDR rips that look great for only 5-7 GB. HD rips can easily be 1-2 GB.

Include VLC if you need a player, but again I’ve found nearly everything plays 265 these days. It’s not nearly the compatibility issue it was years ago.

MP4 container, 265 video (w/ HDR10 layer if appropriate), Dolby digital audio (w/ Atmos track if appropriate) is what I usually look for these days for a balance of compatibility and quality.

.mpreg has the best compression to quality ratio.

Just put a portable copy of vlc on there well